Current aircraft monitoring systems typically use aircraft communications addressing and reporting system (ACARS) data in combination with radar data in order to track the progress of aircraft. This data may be used by air traffic controllers or alternatively provided as a service to aircraft operators.
In the ACARS system, each aircraft is fitted with a VHF transceiver for providing a data link between the aircraft on-board equipment and ground equipment. This data link may be provided through a direct transmission from the aircraft to a ground station, or alternatively the aircraft may transmit the data to a satellite, which then forwards the data to a satellite ground station. These transmissions are received at the ground stations by a data link service provider that then routes the data to the air traffic controllers or aircraft operators.
The periodicity within which a given aircraft will emit ACARS data transmissions is configured by the operating airline and is typically in the order of ten to twenty minutes. This is generally determined in order to provide a balance between receiving up to date data and the per message costs associated with the data transfer. In view of this relatively long period between consecutive message transmissions, the time stamp for any given ACARS transmission is only accurate to within a minute and the position data is reported within accuracy of three decimal places. This means that significant distances can be covered by an aircraft between consecutive ACARS transmissions, which can in turn lead to an uncertainty in the estimated position and path of an aircraft.
Furthermore, if the aircraft is forced to circle in a given area of airspace, for example, in an airport holding pattern, this will not be immediately apparent from the ACARS data as the aircraft will likely have performed a full circle by the time a subsequent ACARS transmission is carried out. This can lead those monitoring the ACARS data to be unsure as to whether these data transmissions are erroneous or if the aircraft truly has remained in a given area of airspace between subsequent ACARS transmissions.
Increasing the frequency (i.e. reducing the period between consecutive transmissions) of ACARS messaging would provide a more up to date set of position data; however, since this ACARS system is a one-to-one digital data link system, this would place a large burden on the ACARS network. This would overload the network, which would then reduce the reliability and accuracy of the network. Accordingly another solution to this problem must be found.
One alternative data source for aircraft position information is to use primary and/or secondary radar installations. Primary radar is an independent method of monitoring the location of a given target aircraft and simply uses the well known principle of emitting a high power radio transmission and then detecting the reflected transmissions from any object that is in the radar's field of view.
In secondary radar, the target aircraft must be fitted with a transponder such that the aircraft can identify itself, in response to an interrogation signal emitted by the radar installation, using a code that has been issued to that aircraft by an air traffic controller. Radar systems have the advantage that aircraft position can be tracked with a greater frequency in order to more accurately monitor a given aircraft's flight path; however, each radar installation requires a very high amount of power in order to transmit the radar pulse over the operational range of the radar system. Furthermore, radar systems are very expensive to install and maintain, especially over large areas.
It has been appreciated by the present applicants that a more accurate system for monitoring aircraft positions that can be implemented using comparatively low cost apparatus is required.